The present invention relates to chemical mechanical polishing. More particularly, the present invention relates to apparatus and methods for chemical mechanical polishing of substrates using a belt.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar.
Chemical mechanical polishing (CMP) is one accepted method of planarizing a substrate. This method typically requires that the substrate be mounted on a carrier head. The exposed surface of the substrate is placed against a rotating polishing pad or moving polishing belt. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface. In addition, a polishing slurry, including at least one chemically-active agent, may be supplied to the polishing pad. Unless a fixed abrasive polishing pad is used, the slurry should also contain abrasive particles.
One problem in CMP relates to slurry distribution. The CMP process is fairly complex, requiring the interaction of the polishing pad, abrasive particles and reactive agent with the substrate. Accordingly, ineffective distribution of the slurry across the surface of the polishing pad provide less than optimal polishing results. Rotatable polishing pads have been used which include perforations about the pad. The perforations, when filled, distribute slurry in their respective local region as the polishing pad is compressed. This method of slurry distribution has limited effectiveness because each perforation in effect acts independently. Thus, some of the perforations may have too little slurry, while others may have too much slurry. Furthermore, there is no way to directly channel the excess slurry to where it is needed.
Another problem in CMP is “glazing” of the conventional (non-fixed abrasive) rotatable polishing pad. Glazing occurs when the polishing pad is heated and compressed in regions where the substrate is pressed against it. The roughened surface of the polishing pad is smoothed out and the perforations in the polishing pad are filled up, so the surface of the polishing pad becomes less abrasive. As a result, the polishing time required to polish a substrate increases. Therefore, the polishing pad surface must be periodically conditioned to maintain a high throughput.
In addition, during the conditioning process, waste materials associated with abrading the surface of the pad may fill or clog the perforations in the polishing pad. Filled or clogged perforations can not hold slurry, thereby reducing the effectiveness of the polishing process.